The story of the Saitu family, a homeless family living in a motel, has led to questions in the New Zealand parliament.
Photograph: Eleanor Ainge Roy for the Guardian

Unicef and children’s charities are urging the New Zealand government to do more to help the most vulnerable in society after a Guardian report highlighted the extent of child poverty in the country.

More than 300,000 children live in poverty in New Zealand – an increase of 45,000 since a year ago and double the number since 1984.

The Unicef country executive director was quoted as saying New Zealanders’ empathy had hardened towards its most vulnerable citizens, and child poverty was becoming “normalised” in the island nation of 4.5 million.

But the government has hit back, saying criticism in the story is not justified.

Responding to questions in parliament, the social development minister, Anne Tolley, dismissed the report.

Labour’s spokesperson for children, Jacinda Ardern, had asked the minister: “What recent publicity does she think led to the Guardian writing an article titled ‘New Zealand’s most shameful secret: we have normalised child poverty’?”

Tolley replied: “I have no responsibility for articles published in the Guardian, a leftwing English newspaper that supports [the leader of the opposition] Jeremy Corbyn.

“To say that a third of New Zealand children can only dream of education and employment is sensationalist rubbish, and I do not think that the measure used in the article is an accurate reflection of poverty in New Zealand at all.”

The figures used to derive the statistic that one-third of New Zealand children live below the poverty line is the definition used by Unicef, Child Poverty Action Group New Zealand and Organisation for Economic Co-operation and Development countries.

New Zealand's most shameful secret: 'We have normalised child poverty'

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Child advocates and politicians called on the government to investigate and accurately measure the extent of child poverty around the country.

Vivien Maidaborn, the executive director of Unicef New Zealand, said since the article had been published visits to the charity’s website and donations had increased by 30%.

“We have had emails and phone calls from colleagues around the world expressing shock at this level of child poverty in New Zealand,” she said.

“They had no idea that the myth New Zealand told about itself as the ideal place for children to live and grow up is only true for some of our children.”

Next month Tolley will travel to Geneva to report on the government’s progress on child welfare to the committee on the rights of the child.

Maidaborn said despite good intentions from individuals, the government was failing to meet its commitments to the UN convention on the rights of the child.

“It will be the government’s chance to demonstrate that from the bottom of their hearts they are turning themselves inside out to do better for New Zealand children … as things stand, they can’t say that.”

Last year, the New Zealand government increased benefits for families by up to NZ$25 (£14) a week.

Although praised by all sides for taking a step in the right direction, Unicef said the increase needed to be four times that to make any dent on child poverty.

“I don’t think we can hold our head high on any international stage regarding this issue,” said Ardern.

“New Zealanders believe in themselves as an egalitarian nation and at the moment that reputation has been dented, but it can be redeemed.

“We still have the potential to be world-leading in this area, we are small enough that we can absolutely introduce child wellbeing policies so that New Zealand is once again a great place to bring up children and be a child.”

The Child Poverty Action Group said the government urgently needed to increase benefits for the most vulnerable families and make the welfare of children a priority.

It said inadequate incomes, expensive and low-quality housing, and poor policies were contributing to an increase in child poverty.

The Green party’s co-leader, Metiria Turei, said she believed child poverty had become so extreme because the government refused to acknowledge the problem for what it was – “a crisis”.

“We are seeing hard-working families living below the poverty line, it is a shameful secret,” she said.

“These are decent New Zealanders who have dignity and work every day to provide for their families. But after years of neglect by the government even their best efforts are not making a difference. The government needs to step up and deal with this crisis immediately.”

The prime minister, John Key, and Tolley were contacted for comment but did not respond.